Transient electromagnetic (TEM) mineral prospecting commonly employs magnetic field gradient detectors (dB/dt detectors such as induction coils. However, it is known that detectors which measure absolute magnetic field intensity (B-field detectors) provide more accurate responses from deep mineral bodies than responses provided by magnetic field gradient detectors. It is also known that B-field detectors provide superior performance to dB/dt detectors in the presence of a conducting overburden, and such conducting overburdens are characteristic of much of the terrain in Australia and elsewhere. Thus, it is advantageous to use B-field detectors in TEM mineral prospecting.
A TEM prospecting system detects the presence of ore bodies by virtue of their higher conductivity than the surrounding ground. Thus, TEM systems also respond to conducting materials such as metals in the system itself, giving rise to a so-called "self response". Thus, to minimise the effects of the self response, TEM magnetometers should preferably contain a minimum amount of metal,
Vibration can be a serious cause of noise in magnetic detectors operating in the Earth's magnetic field. For example, an angular vibration of amplitude 10.sup.-6 radians can cause fluctuations of 50 pT in the geomagnetic field component measured by the detector. Such fluctuations are much larger than the minimum TEM magnetic field to he measured. Thus, TEM magnetometers should preferably be immunised against vibration.
SQUID action can be destroyed by radio frequency (rf) interference from, for example, radio and television transmitters. SQUIDs must, therefore, preferably be shielded against such interference by full of partial enclosure in conducting containers.
Radio frequency SQUIDs are usually magnetically coupled to associated electronics. The magnetic coupling serves two purposes: Firstly, the SQUID must be coupled to an rf tuned circuit to obtain flux bias, and to enable readout of the SQUID; and conventionally, this magnetic coupling is provided by a small rf coil in close proximity to the SQUID. Secondly, SQUIDs are normally operated in a feedback mode called a flux-locked loop which provides an output voltage proportional to an applied or detected flux over a wide dynamic range. Conventionally, current is fed back to the rf coil to cancel the applied or detected flux. High-T.sub.C materials are characteristically hysteretic in their response to sufficiently large magnetic fields. This is particularly true of the devices incorporating Josephson junctions or weak links, such as SQUIDs. It has been found that the use of the rf coil for flux feedback produces a hysteretie response and hence unacceptable operation with signals having a large dynamic range.
various B-field detectors exist, but they usually fail to meet at least one of the following requirements of a TEM detector: high sensitivity; high bandwidth; high dynamic range; high slew rate; insensitivity to large dc magnetic fields; minimal use of metallic components; small size and low weight; and low cost.
Liquid helium-cooled superconducting quantum interference device (SQUID) magnetometer systems are B-field detectors which can satisfy most of the requirements listed above. However, the cost and the difficulty in obtaining, storing, and transporting liquid helium to remote locations makes such liquid-helium cooled SQUID magnetemptor systems impractical for mineral prospecting. A further disadvantage of such systems is that they are bulky and heavy. This makes them difficult to handle and transport, and difficult to use in confined spaces, such as airborne systems, or remote locations.
Thus, SQUID magnetometers which can operate at higher temperatures than that of liquid helium (i.e., high-T.sub.C SQUID magnetometers) are desirable. Unfortunately, high-T.sub.C SQUIDs do not usually have characteristics which are as close to the ideal as the characteristics of conventional SQUIDs. One of the main reasons for this is that magnetic fields usually penetrate high-T.sub.C superconductors to a greater extent than they penetrate low-T.sub.c SQUIDs. The inferior performance of high-T.sub.C SQUIDs caused prior art flux-locked loop feedback circuits in magnetometers to operate unsatisfactorily.
The rf coil used with rf SQUIDs is small and therefore produces a magnetic field which is non-uniform over the area of the SQUID. Thus, when the flux-locked loop operates, it produces a feedback field which cancels the applied field in some areas of the SQUID but not all. The areas where the applied field is not fully cancelled experience a varying magnetic field when the applied field varies. If the applied field has a wide dynamic range, this can lead to hysteretic behaviour in parts of the SQUID, and hence in the SQUID as a whole.
The coil used for feedback to a dc SQUID may similarly produce a non-uniform field and lead to hysteretic SQUID response.